Septic shock appears to result from excessive release of cytokines [e.g., tumor necrosis factor-a (TNF-a), interleukin-2, etc.] and other pro-inflammatory substances [e.g., nitric oxide (NO)] from cells of the monocyte/macrophage lineage in response to infection or lipopolysaccharide (LPS) administration. The production of these cytokines, as well as their action, is mediated by signal transduction events which induce protein tyrosine phosphorylation. Theoretically, inhibition of protein tyrosine phophory-lation may be beneficial in sepsis. These compounds would block the potentially high cytokine production which is dependent on tyrosine phosphorylation. These protein kinase inhibitors would block both activation or production of cytokinase by bacterial products and the effects of cytokines on target cells. Tyrphostins AG 126 and AG 556 are both protein kinase inhibitors and have been shown to improve outcome in small animal models dur-ing both LPS and live bacterial challenge. Further, both AG 126 and AG 556 have been shown to inhibit LPS-induced TNF production from dog peripheral blood mononuclear cells, in vitro. Before human clinical trials, to establish efficacy and safety of these compounds, studies in a large animal model are needed. In collaboration with Dr. Novogrodsky and his colleagues, we evaluated AG 126 and AG 556 in our canine peritonitis model. In a controlled clinical trial in 100 animals over 6 months, AG 556 but not AG 126 significantly improved survival and prevented multiorgan failure during canine septic shock. This therapeutic agent is proceeding to human clinical trials.